Prior art fuel bundles for boiling water nuclear reactors have standard construction. This construction includes a lower tie plate, an upper tie plate, and a matrix of sealed fuel rods supported therebetween. These fuel rods contain nuclear fuel pellets in sealed containment for supporting the required critical reaction for the generation of steam.
The entire fuel bundle assembly between the lower tie plate and the upper tie plate is surrounded by a flow constricting channel. This channel is commonly square in cross section and made of metal (preferably an alloy called zircalloy).
Water moderator passes from the bottom of the fuel bundle to the top of the fuel bundle. Water enters through the lower tie plate within the channel and passes between the upstanding fuel rods. Water and generated steam exit from within the channel between the fuel rods and out through the upper tie plate.
Both the lower tie plate and the upper tie plate have two well known functions.
First, they are the support points for the sealed fuel rods in vertical and upstanding side-by-side relation. Typically, the upper tie plate forms an overlying matrix of fuel rod support points. Into about eight of these support points are placed correspondingly male threaded tie rod end fittings. The tie rods--containing fuel like the remainder of the fuel rods--are threaded at their lower end for corresponding attachment to the lower tie plate.
Likewise, the lower tie plate forms an underlying matrix of fuel rod support points. These underlying support points correspond for the most part to the overlying support points of the upper tie plate. About eight of these support points are threaded with female apertures. They correspond to the overlying apertures in the upper tie plates. Into these threaded support points in the lower tie plates are placed the lower threaded ends of the so-called tie rods. Thus the two tie plates are tied together with the tie rods.
A fuel bundle channel surrounds the fuel rods between the tie plates. This channel confines the required moderator coolant flow to a flow path which is restricted between the tie plates.
The second function of the tie plates is to define a matrix of apertures for permitting fluid flow into and out of the fuel bundle. Specifically, the lower tie plate defines between its discrete fuel rods support points a first matrix of apertures for permitting the inflow of water coolant. This coolant functions in the capacity of moderating or slowing down reaction produced fast neutrons to produce reaction continuing slow or thermal neutrons. At the same time, as the coolant passes upwardly through the fuel bundle within the channel, a portion of the coolant is turned into steam. This steam--and the coolant that is not turned into steam and remains in the liquid phase--must pass out through the upper tie plate. Consequently, the upper tie plate forms its own matrix of apertures in between its matrix of fuel rod support points. This upper tie plate matrix of apertures permits the outflow of the two phase steam water mixture from the fuel bundle.
The fuel bundle must be periodically replaced and/or inspected during so-called "outages" of a reactor. These outages occur when the central steam generating core of a nuclear reactor has its overlying components removed to provide access through shielding water to the core. During such "outages" sections of the reactor vessel core are removed, inspected, and/or replaced. The core, submerged in a radiation quenching bath of water, has the fuel bundles to be replaced for inspection removed by remotely grasping the fuel bundle at a handle or bail. Needless to say, the bail must define, at the top of the fuel bundle, a support point for the entire weight of the fuel bundle in a depending relationship when the fuel bundle is removed. At the same time, the bail must occupy a minimum dimension so as not to interfere with the active lengths of the fuel rods. Thus the bail is held to a clearance to permit bundle handling equipment to grasp the bundle at the bail.
Once the fuel bundle is supported at the bail the entire weight of the fuel bundle is carried through the bail. This weight includes the weight of the fuel rods, the weight of the upper tie plate, the weight of the lower tie plate and the weight of the surrounding channel.
The support of the flow confining channel has heretofore been provided through the upper tie plate. Typically, the lifting bail has been fastened to the upper tie plate at one set of opposite corners on the square sectioned upper tie plate. The channel has been supported at the upper tie plate at the diagonally opposite upper tie plate corners. Thus, during lifting of the fuel bundle from the core, the load of the surrounding channel has passed from the channel to the corners of the tie plate, across the upper tie plate itself in a cantilevered support, and then through the handle or lifting bail to the point of attachment at the tie plate. Naturally, it has been required to build the upper tie plate strong enough to transmit this load.
The construction of the upper tie plate has been further constrained by the requirement that provision be made for in-service life differential expansion of the supported fuel rods. Specifically, such fuel rods during their inservice life become longer by differing amounts. This elongation is due to many factors including radiation induced "growth" or expansion of the fuel rods.
In the prior art, the required dimension for this necessary differential fuel rod expansion had to be provided directly under the points of channel clip attachment to the upper tie plate. Consequently, the points of channel clip attachment and the underlying dimension required for in-service fuel rod differential expansion have interfered one with another. This interference has been solved in the prior art by reducing the overall length of the fuel rods confined within the fuel bundle. While this provides an adequate space for in-service life fuel rod differential expansion, it forced the fuel rod design at the top of fuel bundle to be constrained by the shorter overall fuel rod length.
The reader will understand that substantial portions of the background section herein where they refer to the prior art are selected with the hindsight of the following disclosure. It will be appreciated that the literal maze of design considerations which enter into overall fuel bundle design have been selected only to disclose the relevant considerations which follow from the below disclosed invention.